The structure of the HL-A molecule was studied using sera developed in volunteers immunized against a single HL-A antigen (parent/child immunization with a single HL-A difference). The existence of many determinants or antigen factors (A.F.) was confirmed by absorption-inhibition tests. Blocking antibodies which are not absorbed on the complement are frequently associated with cytotoxic antibodies in the same serum. The molecules of the 1st and 2nd loci are independent; there is no co-capping on lymphocyte surfaces. During immunization between phenotypically-identical related individuals, a non-HL-A humoral and cellular immunity appeared (HL-B?). The activation of lymphocytes by an LD difference is a preliminary to immunization against HL-A antigens. Correlation between the number (1, 2, 3, or 4) HL-A imcompatibilities and kidney graft survival was established in 918 cases of cadaver kidney transplants.